The invention relates to the field of photovoltaic cells, and in particular to a photovoltaic cell having polychromatic diffractive lenses that split and concentrate or focus the solar spectrum, directing the different spectral bands to appropriate cells.
In conventional single-junction solar cells, when a photon of energy higher than the semiconductor bandgap is absorbed, charge carriers are generated. These charge carriers are attracted to different electrodes, generating an open-circuit voltage. When a load is connected to the cell, a current flows, thus generating electrical power. When the photon has energy larger than the band-gap, the extra energy is mostly transferred into the generated free carriers themselves. These so-called “hot-carriers” lose most of this extra energy as heat by collisions with the crystal structure of the semiconductor. Therefore, the energy within a significant portion of the solar spectrum is wasted.
To overcome this limitation, multi junction (or tandem) solar cells where used. In these tandem cells, multiple layers of semiconductor solar cells are grown on a single substrate. The bandgaps of these multiple solar cells are adjusted such that the bandgap energy decreases from top to bottom. Therefore, higher energy photons are absorbed in the top-most layer, while the rest of the spectrum passes unabsorbed. The subsequent layers (or cells) with lower bandgaps absorb the lower energy photons. These multiple cells are connected in series and a cumulative open-circuit voltage is generated across the entire structure.
However, these tandem multi junction cells have several disadvantages. There is some absorption as the light transmits through each multi junction cell, lowering the overall conversion efficiency. In the tandem cell, the sub-cells are connected in series. The lowest current in the series connection limits the current. This significantly reduces the achievable efficiency since the current generated by each cell is not the same. Tandem multi-junction cells require special care in designing the tunnel junction that connect the cells in series. Optical transparency (wide band-gap) and low electrical resistance are incompatible. Furthermore lattice mismatch is a problem for semiconductor tandem cells.
In the tandem cell, it is very difficult to use more than 3 sub-cells. Since a tightly focusing concentrator is used, the active area of the solar sub-cells is greatly reduced. This will decrease the material costs especially of the sub-cell junction materials.